Jaguar Repair: 1987 XJS V12 lacks power when floored, xjs v12, manifold vacuum

Question
I have a 1987 xjs V12 which runs well, but lacks quick acceleration when overtaking or trying to accelerate rapidly, It kicksdown but lacks the expected sudden thrust of power, acceleration is  slow, could this be a problem with the vacuum switch?

cheers
Greg

Answer
Hi Greg,
A lack of good acceleration can be one or more of a thousand things so you need to first confirm that it is not up to par with what it is suppose to do. Keep in mind that the 87 "S" automatic was not a hot rod. The Euro version did run well. I don't have any 1/4 mile drag specs for you but I do know that a older 67 "E" type with a 4.2 six cylinder straight stick will run off from a "S" automatic V-12.

You can check ignition timing, check manifold vacuum under full throttle full load to see that the vacuum does not drop below 5 in. and make a hard run on clean of new spark plugs to see if you can see any signs of an over rich condition. (flat black on the plugs) (also black smoke from the exhaust) a lean condition is a little harder to spot without CO equipment. some of the signs are surging at light throttle settings, overly clean plugs and slightly high engine temp for the load put on the engine.

Howard